Question about _persistent_id in Phoenix LiveView's inputs_for component

Flop.Phoenix implements the Phoenix.HTML.FormData protocol for its meta struct: flop_phoenix/lib/flop_phoenix/form_data.ex at main · woylie/flop_phoenix · GitHub

At some point, an offset option was added to allow users to build more complex form layouts. By setting the option, the :index, :id, and :name fields on the Phoenix.HTML.Form struct are set in to_form/4:

offset_string = Integer.to_string(offset)

%Phoenix.HTML.Form{
  index: offset,
  id: id <> "_" <> offset_string,
  name: name <> "[" <> offset_string <> "]",
  # ...
}

This worked fine with the old inputs_for function that was moved to PhoenixHTMLHelpers. However, this stopped working with Phoenix.Component.inputs_for/1. I can still pass the offset option (or any other option) to make it available in the form data implementation:

~H"""
<.inputs_for
  :let={ff}
  field={@form[:filters]}
  options={[fields: @fields, offset: 5]}
>
  <%!-- ... %>
</.inputs_for>
"""

And, given the offset `5`, the protocol implementation produces the correct values in the `Form` struct:

```elixir
%Phoenix.HTML.Form{
  id: "flop_filters_5",
  name: "filters[5]",
  index: 5,
  # ...
}

However, Phoenix.Component.inputs_for/1 appears to ignore both the index and the id that the protocol implementation produces. The form structs passed to the inner block always start with index 0 and an according ID (although the name is derived correctly from the form struct returned by the protocol implementation):

%Phoenix.HTML.Form{
  id: "flop_filters_0",
  name: "filters[5]",
  hidden: [{"_persistent_id", "0"}, {:field, :email}],
  params: %{"_persistent_id" => "0"},
  index: 0
}

• Here it initializes the index with 0 instead of the index of the form struct: phoenix_live_view/lib/phoenix_component.ex at 8ea6b487de17ea1eb962f1ec37cd99e7182e5e50 · phoenixframework/phoenix_live_view · GitHub
• And here it builds a new ID instead of using the one from the form struct: phoenix_live_view/lib/phoenix_component.ex at 8ea6b487de17ea1eb962f1ec37cd99e7182e5e50 · phoenixframework/phoenix_live_view · GitHub

I noticed that I can initialize the params on the form struct produced by my protocol implementation with a _persistent_id to achieve the previous behavior:

%Phoenix.HTML.Form{
  index: index,
  id: id <> "_" <> index_string,
  name: name <> "[" <> index_string <> "]",
  params: %{"_persistent_id" => index}
  # ...
}

This leads to the correct index being used by inputs_for, but this relies on an internal implementation detail and probably isn’t meant to be used this way.

I don’t know why it was implemented this way, and whether this was an oversight, or what the purpose of the _persistent_id is, or whether I’m doing something wrong. Can someone more familiar with LiveView give me some insights?